儲能元件之靜電紡絲奈米碳纖維/ 二氧化錳奈米複合材料之製備與鑑定

Preparation and Characterization of Electrospun Carbon Nanofibers / Manganese dioxide Nanocomposites for Energy Storage Devices

林聖奇(Sheng-Chi Lin)

指導教授 ： 馬振基 ； 胡啟章

國立清華大學/工學院/化學工程學系/博士(2018年)

摘要

本研究旨在探討超級電容器和靜電紡絲(electrospun)的特性，研究內容以奈米碳纖維(carbon nanofiber, CNF)和二氧化錳(MnO2)所組成電極的製備和性能之關係。主要研究分為以下三個部分:
1.利用聚丙烯腈改質碳纖維電紡絲和二氧化錳組裝成非對稱超級電容器並具高能量和功率密度
利用奈米纖維織物，將CNF改質接枝含氧官能基進而具良好的親水性和孔洞性，並利用其官能基使MnO2均勻沉積在碳纖維上。在這複合材料上展現極佳的電容(415 F g-1 at 5 mV s-1)；充放電一萬圈後，電容仍保有94%。於非對稱超級電容器電位窗能達2.0 V，並具有極高能量和功率密度(36.7 Wh kg-1和354.9 W kg-1)，顯示改質接枝含氧官能基之碳纖維且均勻沉積MnO2之電極，能展現極佳之電化學性能。
2.將鈉離子預嵌入二氧化錳層間與奈米碳纖維組成電極並組裝成非對稱式超級電容器，使其具有高能量及功率密度
本研究首先提出利用簡單、低成本的方法，使鈉離子預嵌入δ-MnO2層中並在奈米碳纖維上生長（命名為NaxMnO2@CNF），應用於非對稱式超級電容器。Na+離子預嵌入於δ-MnO2層狀結構中，雖然會降低結晶度，但有利於Na+離子向層間結構擴散和提高擬電容之MnO2的利用性。而CNF能當作良好的集電器(current collector)，進而和擬電容之MnO2展現協和效應，NaxMnO2@CNF在1 A g-1下展現極佳的比電容值，高達321 F g-1，而掃描速度從1 A g-1到32 A g-1的電容維持率高達75.2％。而由該複合材料和活性炭組成的ASC電池作為正電極和負電極，可在電池電壓為2.0 V時，擁有21 Wh kg-1和1 kW kg-1的比能量和功率。且在10,000次循環充放電測試中，也表現出優異的電池電容維持率（93%），顯示極有潛力應用於儲能材料。
3.奈米碳纖維/鉀離子預插層二氧化錳之高功率和能量密度之可撓式超級電容器研究
本研究中首先提出在奈米碳纖維上生長預嵌入鉀離子的MnO2，用於不對稱超級電容器的正極，並且搭配奈米碳纖維在800 oC（ACNF）下用KOH化學活化負極。通過將鉀離子預嵌入其層狀結構中，MnO2的結晶度顯著降低。這種結構特徵有利於K +擴散進出層間結構中，而有效地利用KxMnO2的電活性材料。這種獨特的複合電極既可提供KxMnO2的理想擬電容特性。在1 A g-1時具有相當高的比電容值279 F g-1，且1 A g-1到32 A g-1的電容維持率約為82.3％。由正極KxMnO2@CNF電極、隔離膜、負極ACNF電極組成的可撓ASC。該電池在2 A g-1的10,000次循環下，電容值仍然維持90%，且比能量和比功率分別達23.5 Wh kg-1和211.4 W kg-1，顯示出優異的ASC性能。再者，在無彎曲和彎曲角度為90°時的電荷存儲行為沒有表現出明顯的差異，展現出有可撓式儲能元件潛力的應用價值。

關鍵字

靜電紡絲 ； 儲能元件 ； 二氧化錳 ； 奈米複合材料 ； 超級電容器

並列摘要

This study focuses on the preparation and performance of the supercapacitor for energy storage devices due to their special characteristics. The research topics of this dissertation are related to the preparation and properties of the electrodes of supercapacitor. There are three parts in this study:
1.Asymmetric supercapacitors based on functional electrospun carbon nanofiber/manganese oxide electrodes with high power density and energy density
Carbon nanofibers modified with carboxyl groups (CNF-COOH) possessing good wettability and high porosity which are homogeneously deposited with amorphous manganese dioxide (amorphous MnO2) by potentiodynamic deposition for asymmetric super-capacitors (ASCs). The potential-cycling in 1 M H2SO4 successfully enhances the hydrophilicity of carbonized polymer nanofibers and facilitates the access of electrolytes within the CNF-COOH matrix. This modification favors the deposition of amorphous MnO2 and improves its electrochemical utilization. In this composite, MnO2 was homogeneously dispersed onto CNF-COOH which provides desirable pseudocapacitance and the CNF-COOH network works as the electron conductor. The composite of CNF-COOH@MnO2-20 shows a high specific capacitance of 415 F g-1 at 5 mV s-1. The capacitance retention of this composite is 94% in a 10,000-cycle test. An ASC cell consisting of this composite and activated carbon as positive and negative electrodes can be reversibly charged/discharged to a cell voltage of 2.0 V in 1 M Na2SO4 and 4 mM NaHCO3 with specific energy and power of 36.7 Wh kg-1 and 354.9 W kg-1, respectively. This ASC also shows excellent cell capacitance retention (8% decay) in the 2V, 10,000-cycle stability test, revealing superior performance.
2.Asymmetric supercapacitors based on electrospun carbon nanofiber/sodium-pre-intercalated manganese oxide electrodes with high power density and energy density
This study first presents that the sodium-pre-intercalated δ-MnO2 is in-situ grown on carbon nanofiber via a simple, one-step method for the application of asymmetric supercapacitors. The pre-intercalation of Na ions into layered structure of δ-MnO2 reduces the crystallinity, beneficial for Na+ diffusion into/out the interlayer structure and pseudocapacitive utilization of MnO2. This NaxMnO2@CNF nano-composite with desirable pseudo-capacitance from δ-NaxMnO2 and high electric conductivity from CNF network shows a high specific capacitance of 321 F g-1 at 1 A g-1 with ca. 75.2 % capacitance retention from 1 A g-1 to 32 A g-1. An ASC cell consisting of this nanocomposite and activated carbon as the positive and negative electrodes can be reversibly charged and discharged to a cell voltage of 2.0 V in 1 M Na2SO4 and 4 mM NaHCO3 with specific energy and power of 21 Wh kg-1 and 1 kW kg-1, respectively. This ASC also shows excellent cell capacitance retention (7 % decay) in the 2V, 10,000-cycle stability test, revealing superior performance.
3.Novel, flexible supercapacitors based on activated carbon nanofiber and carbon nanofiber/potassium-pre-intercalated manganese oxide
In this study, potassium-pre-intercalated MnO2 is grown on carbon nanofibers (KxMnO2@CNF) for the positive electrode of asymmetric supercapacitors (ASCs) and an electrospun CNF is chemically activated with KOH at 800°C (ACNF) for the negative electrode. The crystallinity of MnO2 is significantly reduced by the pre-intercalation of K ions into its layered structure. This textural characteristic is beneficial to the K+ diffusion into/out the interlayer structure, leading to effective utilization of the electroactive material of KxMnO2. This unique composite electrode provides both ideal pseudo-capacitive behavior from KxMnO2 and excellent electric conductivity from the CNF network, exhibiting a fairly high specific capacitance value of 279 F g-1 at 1 A g-1 with ca. 82.3 % capacitance retention from 1 A g-1 to 32 A g-1. A flexible ASC consisting of the positive KxMnO2@CNF electrode, a paper separator, and the negative ACNF electrode is successfully assembled. This cell shows superior ASC performance between 0 and 2 V for 10,000 cycles (10 % decay) at 2 A g-1 with specific energy and power of 23.5 Wh kg-1 and 211.4 W kg-1, respectively. The charge storage behavior of such a cell without bending and with a bending angle of 90o shows no apparent difference, demonstrating its potential in the next-generation flexible energy storage devices.

並列關鍵字

Electrospinning ； Energy Storage Devices ； Manganese dioxide ； Nanocomposites ； Supercapacitor

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